Materials processing in space uses the novel behavior of materials in near zero gravity or microgravity. Unusual microstructures result in such processes due to the absence of container contamination and the reduction of nucleating heterogeneities. Furthermore, the elimination of gravity induced convection may minimize structural defects in the processing of semiconductor materials.
However, zero gravity is not easy to achieve, even in space shuttle flights. Spacecraft trajectory alterations (providing a force approximately 10.sup.-7 g), accelerations associated with atmospheric drag (providing a force approximately 10.sup.-6 g), and astronauts' movements (providing a force approximately 10.sup.-3 g) will lead to relative motion between the levitated specimen and the spacecraft reference frame. Hence, there is a need for an adequate sample positioning control system.
In conventional systems for the containerless processing of materials, the known sample positioning methods (also called sample levitation) use a variety of techniques for generating the requisite force to confine the sample within a predefined zone. The past sample positioning systems for manipulating the position of a sample include: electromagnetic suspension, electrostatic levitation, and acoustic levitation.
The type of force generating mechanism used to levitate a sample depends on the sample's characteristics; i.e., whether it is a metal, nonmetal, or liquid drop. However, the conventional methods cannot be used for the containerless processing of a nonmetallic sample material at elevated temperatures, under vacuum microgravity conditions.
The following table compares the attributes of these various techniques:
______________________________________ Comparison of the various methods for sample positioning Electro- magnetic Active (Eddy- Electro- Magnetic current) Acoustic static Levita- Levita- Levita- Levita- tion tion tion tion ______________________________________ Sample Ferro- Electri- Metallic, Metallic material magnetic cally non- non- conduc- metallic, metallic, tive liquid liquid materials drops drops Control Feedback No servo No servo Feedback require- servo needed needed servo ment Power Small Large Medium Small required (several (--kW) (about (several to mW) 100 W) mW) levitate one gram Sample External High External External heating means degree of means means self- heating Levitation Possible Possible Not Possible under possible vacuum Levitation Not Possible Possible Not of sample possible possible at high tempera- ture ______________________________________
From the table it might be through that acoustic and electrostatic levitation methods would be suitable for the containerless processing of nonmetallic specimens. However, the acoustic technique cannot work under vacuum; and electrostatic levitation would become unstable at temperatures in excess of 600.degree. C. and at vacuum levels greater than 10.sup.-5 Torr. This control instability of electrostatic levitation is due to field-induced emission, anomalous charging mechanisms, and thermionic emission, which prevent levitation at high temperatures.
Furthermore, in these conventional methods listed, the work envelope is directly coupled with the parameters of the force generating mechanism. In the electrostatic levitation method, a limitation on high voltage restricts the interelectrode distance and the amount of sample traverse available. In electromagnetic suspension the coil geometry and the high frequency current also limit the work space.
FIG. 1 shows a Venn diagram that depicts the various combinations of environmental conditions possible where sample levitation might be used. In FIG. 1, the ambient atmosphere is shown by reference numeral 1 and vacuum as reference numeral 2. A high temperature condition is shown as reference numeral 3, the use of a nonmagnetic sample as reference numeral 4, and the use of a nonmetallic sample as reference numeral 5.
By comparing the information within the table above and the Venn diagram of FIG. 1, we can depict the need for a new sample positioning method that is especially suited for high temperature, high vacuum processing of samples in microgravity. This environmental region shown by the pie (reference numeral 6) within the Venn diagram of FIG. 1 is, viz., the levitation of a nonmetallic, nonconductive specimen, at elevated temperatures, under vacuum. The instant invention recognizes that a laser levitation system would be acceptable for levitating a sample within these environmental conditions.
Laser systems have been provided to levitate and position particles within a gravity-oriented vacuum environment. U.S. Pat. No. 4,092,535 to Ashkin et al. discloses a levitation device in which a single laser beam is directed into a vacuum chamber in which a particle is to be levitated.
Recognizing the instabilities inherent within a vacuum oriented laser levitation system, U.S. Pat. No. 4,092,535 includes a feedback system. The feedback system detects the scattered light from the laser beam, which is scattered by the suspended particle, to provide feedback signals. The feedback signals include an error rate feedback signal to control vertical particle deflections and a beam adjustment feedback signal. These signals are of great importance in gravity environments. U.S. Pat. Nos. 3,710,279 and 3,808,550 to Askin et al. use plural laser beams directed at a particle.
In each of these designs, the particle is lifted by directing the laser beam incident specifically focussed upon the particle. Although these designs place the maximum force and momentum of the laser beam upon the particle, it produces large, recognized instabilities.
These levitation designs are akin to placing a ball upon the head of a pin and pushing. The ball is bound to be deflected and fall over. In response to these instabilities, U.S. Pat. No. 4,092,535 provides a computer feedback system. Furthermore, these conventional laser designs cannot provide for the levitation of an entire sample. They can only be used for isolated particles.